Measurement errors in any vector network analyzer (VNA) contribute to the uncertainty of the device being measured by the VNA. By quantifying these errors, their effects can be drastically reduced.
Measurement errors in network analysis can be separated into two categories: random errors and systematic errors. Random errors are non-repeatable measurement variations due to physical change (e.g. noise and temperature changes) and, therefore, are usually unpredictable. Systematic errors are repeatable measurement variations in the test setup itself (e.g. directivity, source match, and the like).
In most measurements made on "devices under test" (DUT) with a VNA, the systematic errors are the most significant source of measurement uncertainty. Therefore, it is desirable to remove these errors from the VNA measurements. This is achieved through a VNA calibration.
It is known in the prior art to connect a number of well-known physical standards (known as mechanical primary standards) to each of the two ports of the VNA for the purpose of calibration. Electrical characteristics of the mechanical primary standards are derived from known physical properties of the standards (e.g. physical dimension, conductor material, and the like). The systematic errors of the VNA can be determined by computing the difference between the VNA measured response of the mechanical primary standards and the known electrical characteristics of the mechanical primary standards.
However, before measuring the DUT, the performance of the calibration should be checked for its accuracy. It is, therefore, also common in the prior art to check the calibration accuracy by connecting another primary standard (a verification standard), which is different than that of the calibration standard, to the VNA. If the calibration of the VNA is performed properly, the measurement of the verification standard closely matches the known electrical characteristics of the verification standard. However, if the measurement of the verification standard does not comply with the known electrical characteristics of the verification standard, then the operator knows that the calibration was not performed properly, or that the VNA is not functioning properly.
Upon completion of verification of the VNA calibration, the operator can then connect the uncharacterized DUT to the VNA for measurement. The systematic error of the measurement system can then be removed mathematically from the measurement of the DUT.
A two-port DUT to be measured can have any of three possible configurations of connectors at its two ports. An "insertable" device has two connectors which are from the same connector family and of opposite sex, one connector being male and the other connector being female. An insertable two-port DUT is configured such that the calibration may be performed by connecting the two ports of a VNA together with the aid of a cable to establish a through-connection, during calibration, and without having to change the configuration of the measurement setup for the actual measurement of the DUT.
In contrast, a reversible DUT to be measured is characterized by two connectors of the same family but also of the same sex (either both male or both female). A reversible DUT is not "insertable" (e.g. "non-insertable") because the two ports of the VNA cannot be connected together to establish a through-connection during calibration without a first adapter. However, disadvantage of this setup is that the adapter becomes part of the calibration measurement. Therefore, it is common practice to calibrate the VNA with the first adapter, which is equal in electrical characteristics to a second adapter, then to switch the first adapter with the second adapter, and then perform the actual DUT measurement. This technique is used in order to try and reduce the measurement uncertainty. However, if the adapters are not equal in insertion loss, amplitude and phase match, and electrical length, then there is an added error in the calibration. Thus, any characteristic variations between these adapters causes added uncertainty in the measurement of the DUT. Alternatively, there is a second non-insertable calibration technique known in the art as "adapter-removal", which provides better calibration accuracy than the "adapter-swap" method described above, and which is shown in FIGS. 2 and 3 and described below.
A second category of a "non-insertable" DUT comprises a transitional device which has two connectors that are of different families (e.g. one connector being coaxial and the other being waveguide). Similar to the reversible DUT, the disadvantage to measurements made on a transitional DUT is that the DUT cannot be inserted into the measurement system using the identical configuration which was used to calibrate the measurement system.
As discussed above, it is common to use a calibration kit including a set of three mechanical primary standards of an appropriate connector family and sex to determine the error coefficients of a predetermined error model of a VNA. These primary standards usually consist of a short circuit connector, a shielded open circuit connector and either a fixed or sliding matched load termination. The fixed and sliding loads are generally mechanical transfer standards. A calibration kit also usually includes several phase-matched adapters for use in the non-insertable DUT "adapter-swap" calibration method as discussed above.
Full two-port calibration using a twelve-term error correction model to determine the systematic errors of a VNA is one of the most comprehensive calibration procedures. In order to determine all twelve terms of the error correction model for an insertable DUT, to be measured, each of the three primary measurement standards of the appropriate sex must be connected to the appropriate VNA port and measured. In addition, the two measurement ports of the VNA must be connected together via the use of a "through" connection.
The calibration setup and required connections to the primary standards for an insertable device are shown in FIGS. 1A and 1B. Thus, an insertable device requires a minimum of six one-port (short, open, load) calibration standards 100, 102, 104, 106, 108 and 110 to be connected in succession to the VNA 112 ports 114 and 116, respectively, (three for each port) and measured, and one through-connection (FIG. 1B) in order to determine the twelve terms of the error correction model.
Alternatively, a non-insertable device requires that an adapter 144, which has the same connector types and sex at each of its ports as the DUT to be measured, and the primary standards be connected to the VNA ports as shown in FIGS. 2 and 3. Thus, the technique requires a minimum of twelve primary standards 120, 122, 124, 126, 128 and 130 (FIG. 2A) and 132, 134, 136, 138, 140 and 142 (FIG. 3A) be connected to the VNA ports 114 and 116 and measured. Furthermore, two through-connections must be established (FIGS. 2B and 3B) in order to perform a full two-port calibration. Thus, referring to FIGS. 2 and 3, this technique requires that the adapter 144 be alternatively connected to each port 114 and 116 of the VNA 112 and a full two-port calibration be performed with the appropriate primary standards. Two calibration sets are then generated and used with the known electrical length of the adapter to compute actual S parameters of the adapter, which is, in turn, used to create a calibration set without the adapter (as if PORT 1 and PORT 2 of the VNA had been actually connected together). Thus, a non-insertable full two-port calibration requires a minimum of twelve primary standard connections and measurements and two through-connections and measurements. However, it is possible to make the two through-connections shown in FIGS. 2B and 3B in succession, thereby reducing the number of through-connections to one.
In addition, for a better accuracy calibration, a sliding termination is typically used instead of a matched load termination. A disadvantage of the sliding termination is that a measurement should be performed for at least three slide positions in order to obtain reliable measurements. Further, it is common in practice to use five slide positions for the matched load measurement at each port thereby resulting in a total of ten matched load position measurements. Thus, for an insertable DUT broadband calibration, a minimum of eighteen measurements and seven connections is standard; and for a non-insertable calibration a minimum of thirty-six measurements and thirteen connections is standard.
A disadvantage of the above-described calibration procedures is that each calibration standard must be connected and measured one at a time. This procedure involves connecting the standard to the VNA port using the appropriate hardware to ensure proper connection and once proper connection is ensured, pressing the appropriate hardware key on the VNA to make the appropriate measurement. In addition, once the measurement has been made, the standard must be disconnected and another standard must be connected using the same procedure. As discussed above, this procedure is repeated for a minimum of seven connections and eighteen measurements with a broadband insertable DUT and a minimum of thirteen connections and thirty-six measurements to measure a broadband non-insertable DUT. Further the electrical length of the adapter must be known in order to use the "adapter-removal" method, or equally matched adapters must be used in order to use the "adapter-swap" method.
As a further disadvantage, a non-trained operator may confuse the standards (which are often confusingly similar in appearance) and operate the wrong hardware key on the VNA, measuring the wrong calibration standard. If the mistake is discovered at the end of the calibration, then the entire calibration must be repeated. Alternatively, if the calibration is not verified by the operator, via the use of a verification standard, after the full two-port calibration, the operator typically does not know that the calibration is flawed and that the DUT measurements are incorrect.
Additionally, the constant connections and disconnections of the calibration standards required by the calibration procedure results in connector and port cable wear and, therefore, non-repeatability in the calibration standard measurements. This non-repeatability in measurements contributes an additional error term to the calibration measurement which cannot be corrected.
Still another disadvantage of the prior art method of calibration is that the manual calibration procedure tends to be cumbersome and slow. Thus, a significant portion of valuable testing time is spent each day calibrating the VNA. If the calibration is not done correctly, then the operator must start over. In addition, the cumbersome calibration is compounded by the fact that the VNA should be recalibrated, depending upon the application, at least once each day in order to ensure appropriate measurement accuracy.
Accordingly, it is an object of this invention to provide a method and apparatus for calibrating a VNA which requires, at most, two connections of the apparatus to any port of the VNA. It is a further object of this invention to provide a method and apparatus that essentially eliminates any errors resulting from connecting the wrong calibration standards to the VNA, and which allows untrained easy calibration of the VNA, while reducing the time required to perform the calibration. The calibration according to this method and apparatus can be performed automatically.